(a) State two characteristics that a medium should have for propagation of sound.

(b) A pendulum has a frequency of 4 vibrations per second. An observer starts pendulum and fires a gun simultaneously. He hears the echo from a cliff after 8 vibrations of pendulum. If the velocity of sound in air is 340 ms^-1, then what is the distance between cliff and observer?

(a) Two characteristics that a medium should have for propagation of sound are:

1. The medium must be elastic so that it's particles may come back to their initial position after displacement on either side, i.e., the particles are capable of vibrating about their mean position.
2. The medium must have inertia so that it's particles may store mechanical energy.

(b) Echo is heard after 8 vibrations of pendulum. Since frequency of pendulum is 4 vibrations per sec, so the time taken for the echo to be heard is t = $\dfrac{8}{4}$ = 2s

If d is the distance between the cliff and the observer, then velocity of sound V = $\dfrac{2\text{d}}{\text{t}}$

Substituting the values in the equation above:

340 = $\dfrac{2\text{d}}{\text{2}}$

d = 340 m

Hence, distance between the cliff and the observer = 340 m